The Global Positioning System (GPS) is a worldwide navigation system that is based on a constellation of earth-orbiting satellites, which are used as reference points to calculate positions on earth. GPS based positioning computations are based upon “triangulation” wherein a GPS signal receiver determines distances to several satellites based upon the travel time of GPS signals transmitted from the corresponding satellites. In addition to determining distances to satellites, GPS receivers may also obtain information from GPS signals indicative of positions of the satellites in space. GPS receivers may also correct for GPS signal transmission delay through the atmosphere and perform other functions.
The integration of GPS receivers with cellular telephones is being driven initially to comply with the emergency location (E-911) mandate of the Federal Communications Commission in the United States. GPS-based position determination technology will also enable location-based applications and value-added services in cellular telephones and other communications devices.
In GPS enabled cellular telephones, it is common for radio frequency (RF) signals transmitted from cellular transceiver antenna to couple with the GPS antenna. As a result, radio energy from the cellular transceiver interferes with the operation of the GPS receiver. The relatively close proximity of the GPS receiver and antenna to the cellular transceiver and antenna in increasingly small handset form-factors favored by consumers aggravates this interference.
In addition to interfering with the reception and decoding of GPS signals while the cellular transceiver is transmitting, GPS receiver operation may also be disrupted while the cellular transceiver is in idle or receive mode. An automatic gain control (AGC) circuit is typically used to adjust the gain of the signal received by GPS antennas to power levels suitable for processing by the GPS receiver. The coupling of RF energy from the cellular antenna to the GPS antenna, however, generally increases the strength of the signal applied to the input of the GPS receiver. In response, the AGC circuit tends to reduce the gain of the GPS signal applied to the GPS receiver. When the cellular transceiver transitions from transmit to idle or receive mode, the gain applied by the AGC module to the signal at the GPS receiver may then be too low. The AGC module must subsequently increase the applied gain in response to the lower power level, but latency associated with controlling the gain of the GPS signal has an adverse effect on GPS signal processing.
All known prior art schemes to blank GPS signals during operation or transmission of radio transceiver signals are based upon sending a blanking signal to the GPS receiver upon detecting the presence of a jamming signal outside of the GPS receiver, before the jamming signal enters the GPS receiver. U.S. Pat. No. 6,107,960 entitled “Reducing Cross-Interference In A Combined GPS Receiver And Communication System” discloses controlling a GPS receiver based on the power of a radio communications power amplifier. Particularly, U.S. Pat. No. 6,107,960 discloses activating or deactivating the GPS receiver front-end and the processing of GPS signals based upon a power level control signal applied to the radio communications power amplifier. Alternatively, U.S. Pat. No. 6,107,960 discloses halting the processing of GPS signals when the power level control signal is high.
U.S. Pat. No. 6,442,375 entitled “Systems And Methods For Maintaining Operation of A Receiver Co-Located With A Transmitter And Susceptible To Interference Therefrom By Sensitization Of The Receiver” discloses desensitizing the GPS receiver to the affects of TDMA signal transmissions. During communication transmissions, an automatic gain control (AGC) module maintains, i.e., prevents the reduction of, gain applied to GPS signals in response to an AGC control logic signal or in response to a communication protocol program signal during TDMA transmission intervals. During idle and receiver modes, the AGC module resumes control of the GPS signal gain. This scheme merely eliminates the latency associated with cyclical GPS signal gain control discussed above, but does not prevent interference of the GPS signal. U.S. Pat. No. 6,442,375 also discloses isolating the GPS receiver from the GPS antenna with RF switch control logic or with a communication protocol program during transmission intervals.
U.S. Pat. No. 6,448,925 entitled “Jamming Detection And Blanking For GPS Receivers” discloses turning off a GPS receiver in the presence of a jamming signal, which is detected by increases in the output of chained correlators or by the presence of clock drift in the absence of temperature changes or by sudden changes in the signal to noise ratio (S/N) of the received GPS signal. U.S. Pat. No. 6,448,925 relies upon dead reckoning methods in lieu of GPS position determination in the presence of a jamming signal.
Eric Hoffman disclosed in a publication at the IEEE Position, Location and Navigation Symposium in 1978 entitled “GPSPAC: A Spaceborne GPS Navigation Set” a method of blanking the input of a GPS receiver with an external blanking pulse to permit GPS operation in the presence of high-power on-board radar altimeter emitters.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below.